Figure 9

Model of Na⁺ secretion mechanism in the posterior rectum of larval A. taeniorhynchus held in high salinity water. A cation-chloride co-transporter, CCC, transporter is the site of Na⁺ entry into the cell across the basolateral membrane. The apical V-type H⁺-ATPase (VHA) pumps protons from the posterior rectal cell into the rectal lumen to establish an electromotive gradient (inside negative). Lumenal protons diffuse down this inward gradient through the apical Na⁺/H⁺ exchanger, NHE3, to drive the secretion of Na⁺ via this exchanger against the large Na⁺ gradient. This H⁺- driven Na⁺ secretion generates the hyperosmotic, Na⁺-rich urine¹⁰.